The present invention relates to a surface elastic or surface acoustic wave (SAW) device which exhibits improved characteristics in the respects of low loss and reduced distortion in the amplitude/phase frequency characteristics over a wide band width.
Concerning a unidirectional electrode device, there has been reported "a flat-amplitude surface acoustic wave filter incorporating a group-based unidirectional interdigital electrode array" designed so as to exhibit low loss and reduced ripple by making use of the unidirectional electrodes. By way of example, reference may be made to "A Collection of Lectures 1-5-14" of Japan Acoustic Academy (by Toshiyasu Meguro et al, Dec., 1976).
In the unidirectional electrode device, an electric phase difference is imparted between two or more input or output electrode groups so that the unidirectional propagation is realized through the interaction of an input or output acoustic wave with geometrical phase difference, to thereby achieve reduction in loss. In the case of the unidirectional electrode device, electric energy inputted to an electrical terminal is converted into surface acoustic wave energy radiated in the direction (forward direction) toward the oppositely disposed input or output interdigital electrode group and/or into a surface acoustic wave radiated in the direction (reverse direction) away from the opposite electrode group. In the course of the following description, the ratio of energy radiated in the reverse direction to that of the forward direction will be defined as a parameter a representative of the directivity, as in the case of Yamada et al's article entitled "Relation of the Insertion Loss and the Triple Echo in SAW Unidirectional Transducer" contained in "JJAP", Vol., 22-3 (1983) suppl. pp. 163-164. More specifically, when the parameter a is "0", this means perfect unidirectional propagation while the parameter a of "1" represents bidirectional propagation.
Heretofore, in the unidirectional electrode device of this type, it is a common practive in the designing that the input or output conductance Ga of a phase shifter circuit and the interdigital electrodes is made to match with an external load conductance Gl at the center frequency and that the directional parameter a is selected as small as possible over a broad band width. However, such design conditions were not necessarily the best conditions, as will be made apparent hereinafter, when considering the characteristics over the whole band.
In the case of the exemplary device disclosed in the aforementioned article, the electrical phase difference provided between the sending electrode and the reflecting electrode for realizing the unidirectional propagation is generated by means of a so-called Bessel type phase shifter which includes resistance elements and inductance elements. Although the Bessel type phase shifter can enjoy the facility in the designing because it requires only two types of circuit elements for construction, the phase shifter suffers a drawback in that the range in which the unidirectional propagation can be obtained in extremely narrow, as a result of which the ripple component is significantly increased at the frequencies deviated from the center frequency.
For having a better understanding of the present invention, discussion will hereat be made on the frequency characteristics of the unidirectional electrode device in which the aforementioned Bessel type phase shifter composed of resistance elements r and inductance elements L is employed. FIG. 11 of the accompanying drawings shows an equivalent circuit of a hitherto known unidirectional electrode device. Refering to the figure, a reference numeral 1 denotes a sending electrode including a resistor 4 and a capacitor 5 and a numeral 2 denotes a reflecting electrode including a resistor 4' and a capacitor 5', wherein the inter-center distance between these electrodes is so selected that the geometrical phase difference .phi..sub.M is equal to (2m.+-.1/2) where m=2, 3, . . . , while a phase shifter 3 as employed is so designed that the electrical phase difference .phi..sub.E at the center frequency is equal to -(.pi./2). FIG. 9 of the accompanying drawings graphically illustrates the loss and frequency characteristics of this unidirectional electrode device. It will be seen that at the center frequency where the conditions for realizing the unidirectional propagation as described hereinafter are satisfied, the directivity (i.e. the ratio of energy propagating in the forward direction as represented by a curve 20 to the energy propagating in the reverse direction as represented by a curve 21') is very significant, while the directivity becomes reduced as the frequency is deviated from the center frequency, resulting in that the ripple component is increased due to the interelectrode multiple reflections or triple transit echo (hereinafter also referred to simply as TTE). FIG. 12 graphically illustrates a relationship between the directivity and the suppression of the TTE. It will be seen that the TTE can be more suppressed as the directivity is increased. In order to realize the TTE suppression of more than 40 dB, the directivity is required to be more than 20 dB, inclusive. In the hitherto known device shown in FIG. 11, the conditions for realizing the unidirectional propagation are given by the equations mentioned below on the assumption that .phi..sub. M represents a geometrical phase leading of the sending electrode 1 relative to the reflecting electrode 2 and that .phi..sub.E represents an electrical phase leading of the electrode located remotest from a power supply source. EQU .phi..sub.M +.phi..sub.E =2.pi., 4.pi., (1) EQU .phi..sub.M -.phi..sub.E =.phi., 5.phi., (2) EQU .vertline.V.sub.1 .vertline.=.vertline.V.sub.2 .vertline. (3)
where a symbol V.sub.1 represents the voltage of the sending electrode and V.sub.2 represents the voltage of the reflecting electrode. The frequency dependencies of the geometrical phase difference .phi..sub.M, the electrical phase difference .phi..sub.E and the ratio .vertline.V.sub.2 .vertline./.vertline.V.sub.1 .vertline., respectively, can be given by the following equations: EQU .phi..sub.M =(2m.+-.1/2).pi.(f/f.sub.o) (4)
where m represents a natural number EQU .phi..sub.E =-.pi./2-2(1+.sub.r G) (.delta.f/f.sub.o) (5) EQU .vertline.V.sub.2 .vertline./.vertline.V.sub.1 .vertline.=1+(.delta.f/f.sub.o) (6)
It will be seen that both of the Equations (4) and (5) are linear approximations of the frequency deviation (.delta.f =f-f.sub.o). FIG. 7 graphically illustrates the frequency characteristics of the geometrical phase difference and the electrical phase difference on the assumption that .phi..sub.M =2.5.pi.(f/f.sub.o), the ratio between the radiation conductance and the capacitive susceptance of electrode is equal to 1 (unity) and that .phi..sub.E =-(.pi./2)-(2.delta.f/f.sub.o). When the Equations (2) and (3) are satisfied and unless the surface acoustic wave propagates in the reverse direction, the directivity can be remarkably increased. However, in the case of the hitherto known phase shifter, .phi..sub.M -.phi..sub.E =3.pi.+(2.5.pi.+2) (.delta.f/f.sub.o)
Thus, the deviation from the value given by the Equation (2) will become steeply increased, as the frequency is deviated from the center frequency.
As will be seem from the foregoing description, the hitherto known phase shifter suffers a drawback in that the frequency range in which the unidirectional propagation can be obtained is extremely restricted, involving a great difficulty in realizing the reduction of ripple over a broad band width. Further, a surface acoustic wave device in which a delay line circuit is employed in place of the phase shifter in an effort to increase the directivity has been reported by R. L. Miller et al (reference may be made to "IEEE, Ultrasonics Symposium Processings", pp. 1-6). However, the delay line is expensive and impractical.
As the phase shifter in which no resistor is employed, there is disclosed in Japanese Unexamined Utility Model Application Publication No. 3614/1983 a phase shifter in which a variable capacity diode is used in combination with a reactance circuit. It is however noted that no consideration is taken as to the frequency band characteristics of the circuit or apparatus to be connected to the phase shifter, not to say of the suggestion for improving the frequency characteristics over a broad band width.